SMT monument phenomenon-quality defects in reflow welding and solutions
In the reflow soldering phenomenon, chip components often stand up.
Cause: The root cause of the monument phenomenon is that the wetting forces on the two sides of the component are unbalanced, so the torque at both ends of the element is not balanced, which leads to the occurrence of the monument phenomenon.
The following conditions can cause an uneven wetting force on both sides of the component during reflow soldering:
▶ The pad design and layout are not reasonable. If the pad design and layout have the following defects, it will cause an uneven wetting force on both sides of the component.
One of the pads on both sides of the component is connected to the ground line or the pad area on one side is too large, and the heat capacity at both ends of the pad is uneven;
The temperature difference across the PCB surface is so large that the heat absorption on both sides of the component pad is uneven;
Large-scale devices such as QFP, BGA, and small chip components around the heat sink will have uneven temperature at both ends.
Solution: Change the pad design and layout.
▶ There is a problem with the printing of solder paste and solder paste. The activity of solder paste is not high or the solderability of the component is poor. After the solder paste is melted, the surface tension will be different, which will cause an uneven wetting of the pads. The amount of solder paste printed on the two pads It is not uniform.The more heat is absorbed by the solder paste on the more side, the melting time lags, and the wetting force is unbalanced.
Solution: Use solder paste with higher activity to improve solder paste printing parameters, especially the window size of the template.
▶ Uneven force in the Z-axis direction when the chip is displaced will cause uneven depth of the component immersed in the solder paste, and the wetting force on both sides will be unbalanced due to the time difference when melting. If the component is displaced, it will directly lead to standing monument.
Solution: Adjust the process parameters of the placement machine.
▶ The furnace temperature curve is incorrect. If the reflow furnace is too short and the temperature zone is too small, the working curve for heating the PCB will be incorrect, resulting in an excessively large humidity difference on the board surface, which will cause an unbalanced wetting force.
Solution: Adjust the appropriate temperature curve according to each different product.
▶ Oxygen concentration in nitrogen reflow soldering. The use of nitrogen reflow soldering will increase the wetting force of the solder, but more and more examples show that the Stand monument phenomenon occurs when the oxygen content is too low, but the oxygen content is generally considered It is most suitable to control it to the negative 6th power of (100 ～ 500) × 10.